Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)

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Transcript of Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR)
Infrared Spectroscopy (IR) Fourier Transform Infrared (FTIR) The
IR Region These frequencies match the frequencies of covalent bond
stretching and bending vibrations.Infrared spectroscopy can be used
to find out about covalent bonds in molecules. type of bonds are
present some structural information The IR Region Infrared
radiation
= 2.5 to 17 m = 4000 to 600 cm1 Units are wave numbers, or cm1,
the reciprocal of the wavelength in centimeters. Wave numbers are
proportional to frequency and energy. Chapter 12 Energy Levels:
Basic Ideas Stretching Frequencies
Frequency decreases with increasing atomic weight. Frequency
increases with increasing bond energy. Chapter 12 Vibrational Modes
Nonlinear molecule with n atoms usually has 3n  6 fundamental
vibrational modes. Chapter 12 Fingerprint of Molecule
Wholemolecule vibrations and bending vibrations are also
quantitized. No two molecules will give exactly the same IR
spectrum (except enantiomers). Simple stretching: cm1. Complex
vibrations: cm1, called the fingerprint region. => Chapter 12
IRActive and Inactive
A polar bond is usually IRactive. A nonpolar bond in a
symmetrical molecule will absorb weakly or not at all. Chapter 12
CarbonCarbon Bond Stretching
Stronger bonds absorb at higher frequencies: CC cm1 C=C cm1 CC
cm1(weak or absent if internal) Conjugation lowers the frequency:
isolated C=C cm1 conjugated C=C cm1 aromatic C=Capprox cm =>
Chapter 12 CarbonHydrogen Stretching
Bonds with more s character absorb at a higher frequency. sp3 CH,
just below 3000 cm1 (to the right) sp2 CH, just above 3000 cm1
(to the left) sp CH,at 3300 cm => Chapter 12 An Infrared
Spectrometer
=> Chapter 12 FTIR (Fourier Transform Infrared)
FTIR spectrometer obtains an infrared spectra by first collecting
an interferogram of a sample signal using an interferometer, FTIR
then performs a Fourier Transform on the interferogram to obtain
the spectrum. Fourier transform defines a relationship between a
signal in time domain and its representation in frequency domain.
FTIR (Fourier Transform Infrared)
An interferometer is an instrument that uses the technique of
superimposing (interfering) two or more waves, to detect
differences between them. Michelson Interferometer. A mirror moves
at a fixed rate. Its position is determined accurately by counting
the interference fringes of a collocated HeliumNeon laser. The
interferometer splits a beam of radiation into two paths having
different lengths, and then recombines them. A detector measures
the intensity variations of the exit beam as a function of path
difference. Schematic of Michelson Interferometer Summary of IR
Absorptions
=> => Chapter 12 An Alkane IR Spectrum => Chapter 12 An
Alkene IR Spectrum => Chapter 12 An Alkyne IR Spectrum =>
Chapter 12 OH and NH Stretching Both of these occur around 3300
cm1, but they look different. Alcohol OH, broad with rounded tip.
Secondary amine (R2NH), broad with one sharp spike. Primary amine
(RNH2), broad with two sharp spikes. No signal for a tertiary amine
(R3N) => Chapter 12 An Alcohol IR Spectrum => Chapter 12 An
AmineIR Spectrum => Chapter 12 Carbonyl Stretching The C=O bond
of simple ketones, aldehydes, and carboxylic acids absorb around
1710 cm1. Usually, its the strongest IR signal. Carboxylic acids
will have OH also. Aldehydes have two CH signals around 2700 and
2800 cm => Chapter 12 A KetoneIR Spectrum => Chapter 12 An
Aldehyde IR Spectrum
=> Chapter 12 OH Stretch of a Carboxylic Acid
This OH absorbs broadly, cm1, due to strong hydrogen bonding.
=> Chapter 12 Variations in C=O Absorption
Conjugation of C=O with C=C lowers the stretching frequency to
~1680 cm1. The C=O group of an amide absorbs at an even lower
frequency, cm1. The C=O of an ester absorbs at a higher frequency,
~ cm1. Carbonyl groups in small rings (5 Cs or less) absorb at an
even higher frequency. => Chapter 12 An AmideIR Spectrum =>
Chapter 12 Carbon  Nitrogen Stretching
C  N absorbs around 1200 cm1. C = N absorbs around 1660 cm1 and
is much stronger than the C = C absorption in the same region. C N
absorbs strongly just above 2200 cm1.The alkyne C C signal is much
weaker and is just below 2200 cm => Chapter 12 A NitrileIR
Spectrum => Chapter 12